It is shown that the RN-AdS black holes have a van der Waals like first order small-large black hole phase transition ending in a critical point [1]}, [2]}. Moreover the phase transition in RN-AdS black hole is remarkable coincidence with van der Waals liquid-gas phase transition, when the cosmological constant and its conjugate quantities are considered as thermodynamical variables [3]}. The cosmological constant corresponds to pressure
\(P=-\frac{\Lambda }{8 \pi }=\frac{3}{8 \pi }\frac{1}{l^{2}}\) 
